1. Field of the Invention
The present invention generally relates to sensors and security system. More particularly, this invention relates to a motions detection system and method for detecting the presence of an intruder by utilizing passive infrared (PIR) motion sensors, which is capable of reducing false alarm due to white light in the security system.
2. Related Art
PIR motion sensors are popularly used as automatic light switches, alarm sensors or to control other electrical appliances. For example, the sensors detect the infrared light emanating from an intruder, such as a human or motor vehicle, and transmit a signal to activate an alarm. The PIR sensors, which are designed to be sensitive to the IR energy produced by the human body temperature range, include a light filter that passes energy between 7 micrometers and 14 micrometers while blocking the remainder. Thus, light in the visible spectrum is either reflected or absorbed by the filter. The energy that is absorbed by the filter causes heating of the filter. This heat is reradiated and is detected by the pyroelectric sensing elements of the sensors.
It is possible therefore for visible light to be converted into IR energy, thereby causing the sensors to issue a false alarm. Thus, PIR motion sensors are inherently susceptible to detecting stimuli not associated with the intruders. Specifically, passive PIR motion sensors are susceptible to the energy produced by automobile headlights and other light resources emanating from outside the region being monitored by the sensors. The energy produced by automobile headlights within a certain distance from the sensors can be sufficient to cause a false alarm issued from the sensors. False alarms in a security system having intrusion detection sensors are a significant distraction to the police force and can be very costly in fines to the owners of the security system.
Traditional approaches to solving this issue include augmenting the out of band energy blocking ability of the pyroelectric detector window/filter with either: 1) adding pigmentation to the lens intended to block “white light” while passing infrared energy (for example, Zinc Sulfide) or 2) the addition of a secondary optical filter. Both approaches have their drawbacks and neither approach satisfies the desire for significantly improved, if not unlimited white light immunity.
U.S. Pat. No. 7,161,152 to DiPoala describes a method and system that provides the potential for unlimited white light immunity, in which a secondary light sensor is utilized to detect when lighting conditions on the motion detector change. If the change in light intensity exceeds a predetermined threshold for this light sensor, the DiPoala system prohibits any potential alarm signal from being generated within a predetermined time period of this change. More specifically, in order to generate an alarm signal, the PIR sensor signal shall exceed a first predetermined threshold for the PIR sensor and, simultaneously, the visible light sensor signal shall not exceed a second predetermined threshold for the light sensor. In other words, the system suppresses sending out an alarm signal if the intensity of the visible light exceeds a predetermined threshold.
Due to foregoing reasons, the DiPoala system does present a potential problem that it is possible for an intelligent intruder to “blind” the motion detector by briefly sweeping a flashlight across the motion detector to shine light upon the detector while he traverses the protected area, thereby suppressing the generating of an alarm signal.
Therefore, it would be very advantageous to provide a motion detecting method and system that provides significantly improved, if not unlimited white light immunity as well as obviates intentional blinding of the sensor by an intruder.